The diiron-oxo proteins have active sites consisting of metal centers bridged by oxo or hydroxo groups usually supported by carboxylate bridges. This expanding class of metalloproteins now includes proteins that perform a variety of functions in biology- -dioxygen transport (hemerythrin), the conversion of ribonucleotides to deoxyribonucleotides (ribonucleotide reductase), phosphate ester hydrolysis (purple acid phosphatases), and oxygen activation (methane monooxygenase, alkane and arene hydroxylases, fatty acyl desaturases, acetylenases, and the ferrioxidase site in ferritins). Both soluble and membrane-bound forms are known. Many of the soluble enzymes have a sequence motif indicative of a carboxylate-rich diiron site, while the emerging membrane-bound subclass appears to have a sequence motif indicative of a histidine-rich diiron site. Oxygen activation at the diiron site is proposed to entail a common mechanism involving diiron (III)-peroxo and high valent iron-oxo intermediates. Building on our past record of modeling structural and spectroscopic properties of such sites, we propose to design and synthesize precursor complexes that would afford such intermediates and to characterize the structural, spectroscopic, and reactivity properties of these reactive species. We propose to generate intermediates such as O2 adducts of diiron (II) complexes and species with Fe(III)Fe(IV) and Fe(IV)Fe(IV) formal oxidation states using a number of ligand design strategies and low temperature methods to increase their lifetime. These complexes will be characterized by x-ray crystallography whenever possible and by a variety of spectroscopic techniques such as NMR, EPR, UV-vis-NIR,Raman Mossbauer, electrospray mass spectrometry, and EXAFS for comparison with corresponding enzyme intermediates. The reactivities of these transient complexes will be studied for their ability to carry out the range of transformations encompassed by this group of enzymes.